Semantic network with selective indexing

ABSTRACT

Indexing overhead in a semantic network is reduced by omitting some links from indexes in semantic network elements in response to one or more omission rules. Inference strategy over the semantic network is modified to prioritize inference strategies in such a way that the impact of the omissions on inference results is minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON ATTACHED MEDIA

Not Applicable

TECHNICAL FIELD

The present invention relates to knowledge processing systems,particularly data representation using semantic networks.

BACKGROUND OF THE INVENTION

Semantic networks are an old and well-known data representation method.A recent overview of the field and a detailed description of aparticular semantic network can be found in H. Helbig: KnowledgeRepresentation and the Semantics of Natural Language, Springer, 2006,which is hereby incorporated herein in its entirety.

A semantic network comprises nodes, typically representing classes andindividuals, and links between nodes (or built-in values such as numbersor strings). Links may be unary, binary, ternary, or of other arities.In some semantic networks links may also refer to other links.

Known semantic network based knowledge representation systems have beenrelatively small scale, with up to some millions or tens of millions ofnodes or links. However, for a large scale knowledge processing system,the semantic network may need to scale to billions of nodes, most ofwhich describe individual objects and individual events.

Current semantic networks do not scale well to such sizes. The presentinvention aims to improve the scalability of semantic network basedknowledge representation systems and to improve their performance.

BRIEF SUMMARY OF THE INVENTION

In known semantic network systems, a link is reachable from any nodethat the link references. That is, links can be followed in anydirection.

In contrast, according to the present invention, some links are madeunidirectional (or not reachable from one or more of the nodesreferenced by the link). This approach goes against traditional wisdomand the assumptions of traditional inference mechanisms, which mayevaluate predicates, subgoals or quantifiers in any order (subject toconstraints due to extralogical primitives in logic programs). If somelinks can be followed in only one direction, then a different querystrategy that orders operations in such a way that links are onlyfollowed in the correct order may need to be used. This adds furthercomplexity to the already difficult problem of making inferencesefficiently in very large knowledge bases.

There are many ways to implement making some links unidirectional. Inthe preferred embodiment, the decision to make a particular linkunidirectional or partially unidirectional is made fully automaticallybased on a set of omission rules. If only a few links reference a node,then the link is likely to be highly relevant for that node. On theother hand, if very many links (especially of the same type) reference anode, then those links are not likely to be particularly relevant forthe description of the concept represented by that node. For example,all the links characterizing individual objects as “red” (the color) arelikely to be relevant for those individual objects, but not for thedescription of the concept “red” (which has very many links).

Nodes in semantic networks typically comprise index data structures thatcan be used to find the links that reference each node. To implementsuch unidirectional links, some links are omitted from the index of thenode at one end of the link. In some embodiments, all links are added,but for some arguments of a link, if the index grows too large or hastoo many links of a particular type, older and less prominent links areremoved from the index. The treatment of arguments may be configured ona per link type (link class) basis. For some links, the first argumentis always indexed (i.e., not lossy), whereas other arguments are lossy(automatically forgotten from the respective nodes after many more linkshave been added, unless something makes the link “prominent”).

There are some important benefits to be gained from such approach thatoutweigh the added complexity in many embodiments. First, indexmanagement overhead is reduced (especially if links are not added to theindex of very frequently referenced objects at all) and the size of theindexes is reduced, reducing memory consumption, garbage collectionoverhead, and the time to read/write the knowledge base to disk ordatabase. Also, if indexes are kept relatively small, more efficientand/or simpler data structures can be used for the indexes.

Also, it turns out the added complexity in queries is mostly anillusion. In an interactive system with response time limits it is notpractical to follow thousands or millions of links during a query.Instead, queries must be organized in such a way that only a relativelysmall number of links are followed. Queries that traverse very largeindexes are not going to be fast enough anyway. Thus, the change in theway queries must be made is actually much smaller than it initiallyseems, and largely parallels what needs to be done in general to makeinference in semantic networks scale.

Also, many knowledge processing systems may operate for extended periodsof time, continuously processing documents, messages, web pages, orother data. If everything the system has ever seen was fully remembered,the system would eventually run out of memory. It is thus necessary toimplement forgetting in such systems in some way. The present inventionprovides one way of doing that.

A further important issue in knowledge bases is clustering data suchthat objects that are semantically related are stored together, toimprove cache locality, and to permit construction of semantic unitsthat can be read or written to disk as one unit or handled indistributed object systems as one unit. Some garbage collectors can alsomake use of such units to optimize garbage collection performance.

If a link is referenced from all nodes referenced by the link, then itmay be difficult to know with which node to cluster the link. However,if the link is referenced from only one node, it is natural to clusterthe link with that node. In this manner, the present inventionautomatically causes links to be clustered with the concepts that theydescribe, rather than, e.g., the attribute values or class objects thatare used as values in the description.

A first aspect of the invention is a computer comprising a semanticnetwork stored in its accessible memory, said semantic networkcomprising semantic network elements, the computer comprising:

-   -   a first omission rule means; and    -   a first index maintainer responsive to the first omission rule        means, the first omission rule means causing the first index        maintainer to omit at least one link referencing a first node        from a first index associated with the first node.

A second aspect of the invention is a computer that further comprises aninference system responsive to the first omission rule means, the firstomission rule means causing the inference system to prioritize aninference strategy that avoids trying to follow, from the first node,links that have been omitted from the first index, over an inferencestrategy that tries to follow such links.

A third aspect of the invention is a method of maintaining an index in asemantic network element, comprising:

-   -   adding, by a computer, a plurality of links into an index        associated with a semantic network element; and    -   omitting, by the computer, at least one link from the index        based on a first omission rule.

A fourth aspect of the invention is a method that further comprisesmaking inferences over the semantic network, prioritizing an inferencestrategy that avoids trying to follow links from indexes from which somelinks have been omitted over a strategy that tries to follow links fromsuch indexes.

A fifth aspect of the invention is a computer program product stored ona computer readable medium, the product operable to maintain an index ina semantic network element, and the product comprising:

-   -   a computer readable program code means for causing a computer to        index a plurality of links into an index associated with a        semantic network element; and    -   a computer readable program code means for causing the computer        to omit at least one link from the index based on a first        omission rule.

A sixth aspect of the invention is a computer program product furthercomprising a computer readable program code means for causing thecomputer to make inferences over the semantic network, prioritizing aninference strategy that avoids trying to follow links from indexes fromwhich some links have been omitted over a strategy that tries to followlinks from such indexes.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 illustrates a computer according to an embodiment of theinvention.

FIG. 2 illustrates adding a link to the index by an integrated indexmaintainer—omission rule means.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that the aspects and embodiments of the inventiondescribed herein may be used in any combination with each other. Severalof the aspects and embodiments may be combined together to form afurther embodiment of the invention, and not all features, elements, orcharacteristics of an embodiment necessarily appear in otherembodiments. A method, a computer, or a computer program product whichis an aspect of the invention may comprise any number of the embodimentsor elements of the invention described herein. Separate references to“an embodiment” or “one embodiment” refer to particular embodiments orclasses of embodiments (possibly different embodiments in each case),not necessarily all possible embodiments of the invention. Where anelement is specified, the intention is that more than one such elementcould also be present. First, second, etc., entities may or may notrefer to the same entity.

FIG. 1 illustrates a computer according to an embodiment of theinvention. A computer may be a general or special purpose computer,workstation, server, laptop, handheld device, smartphone, wearablecomputer, embedded computer, a system of computers (e.g., a computercluster), distributed computer, computerized control system, processor,or other apparatus with data processing capability.

(100) illustrates one or more processors in the computer. The processorsmay be general purpose processors, or they may be, e.g., special purposechips or ASICs. (101) illustrates the main memory of the computer. (102)illustrates an I/O subsystem, typically comprising mass storage (such asmagnetic, optical, or semiconductor disks, tapes or other storagesystems, RAID subsystems, etc.; it frequently also comprises a display,keyboard, speaker, microphone, camera, and/or other I/O devices). (103)illustrates a network interface; the network may be, e.g., a local areanetwork, wide area network (such as the Internet), digital wirelessnetwork, or a cluster interconnect or backplane joining processor boardsand racks within a clustered or blade-based computer. The I/O subsystemand network interface may share the same physical bus or interface tointeract with the processor(s) and memory, or may have one or moreindependent physical interfaces. Additional memory may be located behindand accessible through such interfaces, such as memory stored in variouskinds of networked storage (e.g., USB tokens, iSCSI, NAS, file servers,web servers) or on other nodes in a distributed non-shared-memorycomputer.

(104) illustrates a semantic network in the computer's accessiblememory. The semantic network comprises nodes (105) and links (106).Links have one or more arguments that may reference nodes, in someembodiments other links, built-in data types (e.g., strings, numbers),or various other data representations (e.g., pictures or sounds in someembodiments). Together, the nodes and links are called semantic networkelements.

At least some of the nodes of the semantic network have an index (113)associated with them. The index can be used to find the links thatreference the node. In its simplest form, it may be just a list orarray; however, for nodes that have many links (of different types),more complex data structures such as a search tree or a hash table maybe used. The format of the index may also depend on the number of linksin it; for example, if the index is empty, it could be a NULL pointer;if it contains only one link, it could be just a pointer to that link;if it is relatively small, it could be a vector (array) of pointers tolinks (or a vector comprising link types and pointers to the respectivelinks), and its format could change to, e.g., a hash table keyed by thelink type or some other suitable value if it becomes very big. Each slotin the hash table could again change from a single link to a vector to amore complex index.

In some embodiments of the present invention, some links may also beassociated with a similar index (114). Both nodes and links may alsohave more than one index associated with them, for example an index forlinks referencing the node in “arg1” position, another for “arg2”position, and so forth (or, e.g., one for “arg1” and another for all theothers). (Some semantic network implementations do not call this datastructure an index, but still have a corresponding data structure thatcan be used to find links referencing a node.)

According to the present invention, at least some of the indexes do notcomprise all the links referencing the associated node. In other words,some links may be omitted from the indexes based on one or more omissionrules. This is in contrast to the known prior art, where the ability tofollow the links between nodes has been seen as one of the majorbenefits of semantic networks. Well known inference mechanisms such asspreading activation depend on this in their normal implementations.Also, known logical inference mechanisms assume that inference canfollow the graph in any direction.

Omitting the links is made feasible by using one or more omission rulesthat affect both the maintenance of the indexes and the inferenceprocess. The indexes are maintained by an index maintainer (107), whichmay be implemented, e.g., using specialized circuitry on an ASIC orusing a program code means stored in the computer's memory. The indexmaintainer is coupled to one or more omission rule means (108), whichdecides for each link whether it is to be included in each of theindexes associated with the nodes or links that it references. In someembodiments omission rules may also signal to the index maintainer thatone or more links already existing in an index should be dropped fromthe index when adding another link to the index, and this is one way ofomitting links from an index. In general, maintaining an index includesadding, removing, and modifying values in the index such that the indexis kept up to date with changes to the semantic network.

The omission rule means may also be integrated with the index maintainerso that they form a single logical circuit or program code means. Suchintegration is preferable in embodiments where no dynamic learning ofomission rules takes place. However, in embodiments where the omissionrules change dynamically, it may be preferable to separate the indexmaintainer and the omission rule means into separate modules.

The omission rule means is also coupled to the inference system (109),which makes inferences over the semantic network. The inference systemis responsive to the omission rule means, and in an inference processprioritizes inference strategies that avoid using indexes in such a waythat the omitted links would need to be found from an index. Forexample, in finding the individual that is “Mike's car”, and assumingthat possession is indicated by a two-argument link POSS(OWNER, OBJECT)and the omission rule applicable to “car” causes POSS links referencing“car” by their second argument to be omitted (or at least some of themto be omitted), the inference system would prioritize a strategy thatstarts from “Mike”, and then searches for POSS links where “Mike” is thefirst argument, over searching for POSS links having “car” as theirsecond argument.

Again, if the omission rules are static, the effect of omission rulesmay be integrated into the inference means by taking into account theinfluence of the rules in the inference means. However, if the rules aredynamic, then it may be preferable to keep them separate from theinference means.

(110) illustrates a garbage collector for reclaiming links and nodesthat are no longer reachable from the externally referenced roots of thesemantic network. The garbage collector may use any known garbagecollection technology, including but not limited to reference counting,mark-and-sweep collection, copying garbage collection, generationalgarbage collection, the train algorithm, or a region-based garbagecollector. The implementation of garbage collectors is described indetail in R. Jones and R. Lins: Garbage Collection: Algorithms forDynamic Memory Management, Wiley, 1996.

In the preferred embodiment, the system is configured such that if alink is dropped from an index by the index maintainer, and the link isno longer in any index, and there is no other (live) link referencingthe link, then the garbage collector will automatically reclaim the link(even if it is a part of a garbage cycle, as is known in the garbagecollection literature). One requirement for this to happen is that theremust not be any references to links besides those occurring through thelinks. In particular, there can be no global table or hash tablereferencing the links, unless such table or hash table is weak (as theterm is known in the garbage collection and programming languageliterature). In distributed systems this also applies to stubs/scions orother representatives of remote objects. In many embodiments the garbagecollector may also reclaim nodes that are no longer referenced by any(live) link. Together omitting/dropping links and garbage collectionprovide a means for implementing forgetting in knowledge processingsystems.

(111) represents a clusterer, that is, a mechanism that causes relatedobjects to be clustered near each other in the address space of thecomputer, or into the same computing node in a distributed computer.

Clustering is significantly influenced by omitting links from the indexof some of the nodes that the link references. Essentially the link isno longer referenced from the node from whose index it was omitted. Itwill thus naturally get clustered with the node at the other end of thelink (or one of its arguments, if it has more than two arguments).

Preferably the omission rules are designed in such a way that they causemost links referencing very common concepts, such as common attributes(e.g., colors), common classes in an ontology, etc., to be omitted ordropped. Thus, while the system might know thousands of people, it mightnot be able to enumerate everyone it knows, yet it would know for eachof them that it is a person. If the omission rule specifies that only acertain number of links (of a particular type) be kept in the index,then the system might be able to enumerate a few (or a few dozen)people, but not all of them. In each case (except possibly for the oneskept in the index at “person”) the link would get clustered with theindividual, rather than the class “person”. Preferably in such systemsthe individuals that are kept in the index at “person” would be in somemanner “more prominent” than other links. Prominence could mean thatthey are individuals that are referenced more often than otherindividuals, or that have the most influence on the systems operation,or that are considered “well known” to the people in general. Prominencemay be automatically estimated, e.g., by having the system collectstatistics on how frequently each individual is used in an inferenceprocess, and using these statistics as a criterion in an inference rule.

(112) illustrates an omission rule learner, which is a subsystem forautomatically learning and adjusting omission rules. It may create,delete, or modify existing omission rules.

FIG. 2 illustrates an integrated index maintainer and omission rulemeans according to an embodiment of the invention. Starting from (200),the flowchart illustrates the actions taken when adding a link thatreferences a node. In this example, the omission rules include neveromitting except for “lossy arguments”, and then only omitting if theindex is not too small. Roughly the steps (201) to (204) relate to theomission rule means. Step (201) reads information about the link class(or the information could be statically configured on a per link typebasis if only a fixed set of links exists), (202) determines whether theargument of the link that refers to the node that the link is associatedwith should be treated as “lossy”. If not, then execution moves directlyto adding the link to the index. If the index is “lossy”, meaning thatsome links may be omitted from the index, execution proceeds to (203) tocheck if the index is so small that links should not yet be omitted fromit. When coming to (204), it has been determined that the index shouldnot grow, and (204) decides whether the current link should be omittedor whether some other link should be dropped from the index beforeadding the new link. (205) selects an existing link in the index anddrops it from the index. (206) adds the new link to the index. (207)indicates the end of the operation.

Omission rules may use a variety of criteria to decide whether to omit alink from an index. Some examples of possible criteria include:

-   -   references by some arguments (e.g., arg1) are treated as not        lossy, and other arguments (arg2, arg3, . . . ) are treated as        lossy    -   link class specifies which arguments are to be treated as lossy    -   size of the index    -   age of the nodes referenced by the link (e.g., very new things        not so easily omitted)    -   number of times the nodes referenced by the link have been        accessed.

The various omission rules may be combined, and may have a precedenceorder specified among them.

When the rules trigger an existing link to be dropped from the index,the selection of the link may be based on any suitable criteria, suchas:

-   -   random replacement    -   weighted random replacement, so that more recently added ones        are more likely to be replaced, possibly combined with promoting        frequently accessed ones to be treated like older ones    -   replacing the one that has not been used for the longest time    -   replacing the one leading to a node that has not been visited        for the longest time    -   selecting one that is “not prominent” under any suitable        definition of “prominent” (various examples were given elsewhere        in this disclosure).

The inference system may be based on any known inference system forsemantic networks. It may be a spreading activation system or it may bea logical inference system (e.g., theorem prover for some suitablelogic). Inference may be, e.g., monotonic, non-monotonic, inductive, orabductive. Goal-oriented inference systems are used in many logicprogramming languages, and are suitable for use with semantic networks.Various implementation details for goal-oriented inference systems canbe found in P. Kacsuk: Execution Models of Prolog for ParallelComputers, MIT Press, 1990, and G. Gupta et al: Parallel Execution ofProlog Programs, ACM Transactions on Programming Languages and Systems,23(4):472-602, 2001, which are hereby incorporated herein by reference.

Goal-oriented inference systems typically construct (implicitly orexplicitly) and-or trees of goals that need to be solved. In pure logic,the alternatives in an and-node or an or-node can be evaluated in anyorder. With practical logic programming languages such as Prolog thatinclude extralogical primitives, such as I/O, additional constraintsmust be placed on the evaluation of alternatives at and-nodes andor-nodes, as is known in the art, to preserve the execution order ofsuch extralogical primitives.

In an exemplary embodiment, omission rules are taken into account in agoal-oriented inference system as additional evaluation orderconstraints. Any goal that contains no variables can be evaluated at anytime. Goals with open variables are preferably only evaluated after allother possible substitutions and evaluations have been made, and arethen evaluated starting from any constants in argument positions fromwhich links are not dropped in the applicable index. When no suchstarting points are present, any available constant can be selected as astarting point for evaluation (i.e., link following), but in this lastcase the result may not be complete (in the logical sense). If theselected node (constant) has at least some (preferably highly prominent)links in the index, then values reachable through those links may beconsidered by the inference procedure as possible answers. By evaluatingalternative goals in such an order the inference mechanism prioritizesinference strategies that avoid trying to follow links from indexes fromwhich some links relevant for the query have been omitted overstrategies that try to follow links using such indexes.

The omission rule learner dynamically learns or adjusts omission rules.It may create new rules, delete existing ones, or modify existing rules.In some embodiments it only supports some of these operations. Forexample, in some embodiments the omission rule learner might only adjustspecific parameters in otherwise hard-coded omission rules. It could,for example, collect statistics about how frequently a particular indexis used (e.g., what link types are followed through it) or how a linktype is used in inferences starting from a particular class of node andparticular argument position, and if after some time it seems that suchindex or such links are never searched starting from some class of anode, an omission rule could be created that causes links of that typenot to be added to the index in that class of node for that particularargument.

Many variations of the above described embodiments will be available toone skilled in the art. In particular, some operations could bereordered, combined, or interleaved, or executed in parallel, and manyof the data structures could be implemented differently. When oneelement, step, or object is specified, in many cases several elements,steps, or objects could equivalently occur. Steps in flowcharts could beimplemented, e.g., as state machine states, logic circuits, or optics inhardware components, as instructions, subprograms, or processes executedby a processor, or a combination of these and other techniques. The bookby Kacsuk (1990) provides information on how to implement logicalinference using parallel computers and special hardware.

Computer-readable media can include, e.g., computer-readable magneticdata storage media (e.g., floppies, disk drives, tapes, bubblememories), computer-readable optical data storage media (disks, tapes,holograms, crystals, strips), semiconductor memories (such as flashmemory and various read-only or non-volatile memory technologies), mediaaccessible through an I/O interface in a computer, media accessiblethrough a network interface in a computer, networked file servers fromwhich at least some of the content can be accessed by another computer,data buffered, cached, or in transit through a computer network, or anyother media that can be read by a computer.

1. A computer comprising a semantic network stored in its accessiblememory, said semantic network comprising semantic network elements, thecomputer comprising: a first omission rule means; and a first indexmaintainer responsive to the first omission rule means, the omissionrule means causing the first index maintainer to omit at least one linkreferencing a first node from a first index associated with the firstnode.
 2. The computer of claim 1, further comprising: an inferencesystem responsive to the first omission rule means, the first omissionrule means causing the inference system to prioritize an inferencestrategy that avoids trying to follow, from the first node, links thathave been omitted from the first index, over an inference strategy thattries to follow such links.
 3. The computer of claim 1, wherein: linkscan reference other links; a second link has a second index associatedwith it, the second index comprising references to the second link byother links; and the second index is maintained by a second indexmaintainer, which is responsive to a second omission rule means, thesecond omission rule means causing the second index maintainer to omitat least one link referencing the second link from the second index. 4.The computer of claim 1, wherein the index maintainer implementsomitting at least one link by dropping the link from the first indexwhen signalled to do so by a second omission rule.
 5. The computer ofclaim 4, wherein the selection of the link to drop is responsive to therelative prominence of the links in the first index.
 6. The computer ofclaim 4, wherein a link dropped from the first index is reclaimed by thegarbage collector if it is no longer present in any index and no otherlink references it.
 7. The computer of claim 1, wherein the firstomission rule is responsive to the argument position in the link thatreferences the associated node.
 8. The computer of claim 1, wherein theomission rule is responsive to statistics collected about use of linksin inference.
 9. The computer of claim 1, further comprising an omissionrule learner configured to modify at least one omission rule.
 10. Amethod of maintaining an index in a semantic network element,comprising: adding, by a computer, a plurality of links into an indexassociated with a semantic network element; and omitting, by thecomputer, at least one link from the index based on a first omissionrule.
 11. The method of claim 10, further comprising: making inferencesover the semantic network, prioritizing an inference strategy thatavoids trying to follow links from indexes from which some links havebeen omitted over a strategy that tries to follow links from suchindexes.
 12. The method of claim 10, wherein omitting at least one linkis implemented by dropping it from the index when signalled to do so bya second omission rule.
 13. The method of claim 10, further comprising:modifying at least one omission rule based on information collectedabout the use of indexes in inferences.
 14. A computer program productstored on a computer readable medium, the product operable to maintainan index in a semantic network element, and the product comprising: acomputer readable program code means for causing a computer to index aplurality of links into an index associated with a semantic networkelement; and a computer readable program code means for causing thecomputer to omit at least one link from the index based on a firstomission rule.
 15. The computer program product of claim 14, furthercomprising: a computer readable program code means for causing thecomputer to make inferences over the semantic network, prioritizing aninference strategy that avoids trying to follow links from indexes fromwhich some links have been omitted over a strategy that tries to followlinks from such indexes.
 16. The computer program product of claim 14,further comprising: a computer readable program code means for causingthe computer to implement omitting at least one link by dropping it fromthe index when signalled to do so by a second omission rule.
 17. Thecomputer program product of claim 14, further comprising: a computerreadable program code means for causing the computer to modify at leastone omission rule based on information collected about the use ofindexes in inferences.